The invention relates to an improved process for the preparation of a copolyether ester. Copolyether esters are thermoplastic elastomer polymers built up of hard polyester segments of repeating units derived from at least one alkylene glycol and at least one aromatic dicarboxylic acid or an ester thereof and soft segments derived from a polyalkylene oxide glycol. Such a copolyether ester is generally prepared by a process involving the combining in the melt of at least one alkylene glycol, at least one aromatic dicarboxylic acid or an ester thereof and the polyalkylene oxide glycol. If an ester of an aromatic dicarboxylic acid is started from, for instance the dimethyl ester of terephthalic acid, then first a transesterification reaction takes place upon which the alkylene glycol and the polyalkylene oxide glycol take the methyl position in the aromatic dicarboxylic acid ester, with the methanol, which is volatile under the transesterification reaction conditions, being separated off. If the aromatic dicarboxylic acid is present in place of the ester, then esterification with the glycols takes place directly. Subsequently, polycondensation of the ester to yield polyester, in the case specified here copolyether ester, takes place under reaction conditions that are generally different from those of the transesterification or esterification. The polycondensation in the melt is then continued until a polycondensate with the desired molecular weight is obtained.
In a number of cases, especially if the polyalkylene oxide glycol is based on propylene oxide, the polycondensate must then be subjected to solid-phase post-condensation in order to achieve a sufficiently high molecular weight. For the softer copolyether esters the polycondensation is also much slower than for the harder copolyether ester that contain less soft segment.
It has proved possible to shorten the time needed to obtain a desired molecular weight in the melt condensation process by using a catalyst. Various catalysts have been developed for this; in practice, complex titanium compounds, in particular titanium tetrabutoxide (TBT), have found the widest application.
In U.S. Pat. No. 3,801,547 A and U.S. Pat. No. 4,687,835 A besides TBT salts of a bivalent metal, in particular magnesium acetate and calcium acetate are used as cocatalysts. In said patent publications other combinations of titanium and magnesium are also mentioned, for instance Mg[HTi(OR6]2 where R=alkyl, and other complex titanates obtained from alkaline earth metal alkoxides and titanate esters. No reasons are given for the use of such combinations of a bivalent metal compound with the titanium compounds. The molar ratio of titanium to bivalent metal is generally 2:1.
In spite of the presence of said catalyst combinations, the state of the art processes take quite some time or lead, as for instance stated in Example 2 of U.S. Pat. No. 3,801,547-A at most to a copolyether ester having a minimum melt flow index (MFI) of 5.1 g/10 min. Polyether esters with such a high MFI can be used for only a limited number of processing techniques.
By using chain branching, for instance by alcohols or acids with a functionality of three or higher, for instance trimethylol propane or trimellitic acid, the time needed to reach a certain molecular weight can be shortened also, or a copolyether ester with a lower MFI can be obtained, see U.S. Pat. No. 4,205,158-A. However, the resulting branched copolyether esters have inferior elastic and fatigue properties, making them less suitable for use in, for instance, bellows in automotive applications under more extreme conditions.
Another process by which the problem of the low reaction rate in the preparation of the softer copolyether ester types can be obviated comprises partial replacement of the terephthalic acid by isophthalic acid, so that a lower polyalkylene oxide segments content is needed for a certain shore D hardness, while the hard segments content, which promotes a higher polycondensation rate, increases. However, this process has the drawback that the melting point of the copolyether ester is substantially lower than that of the corresponding copolyether ester that is based entirely on terephthalic acid, while moreover the glass transition temperature is higher. Particularly in applications at higher temperatures and extremely low temperatures, for instance under the bonnet, these iso- and terephthalic acid based copolyether esters prove less suitable. In addition, the elongation at break is lower.
The aim of the invention was therefore to find a process that offers the advantage of an increased polycondensation rate while it does not have the above-mentioned drawbacks, or only to a very limited extent.
The inventors have now found, very surprisingly, that when the ratio of titanium to bivalent metal in the catalyst combination is chosen to be substantially lower than the value of 2 that has so far been customary, for instance 1.6 or even lower, the polycondensation time for a given viscosity is substantially shortened and it proves possible to produce, without solid-phase post-condensation, copolyether ester that is suitable for, inter alia, injection moulding applications, but contains substantially less chain branching agent, or even no chain branching agent at all, than copolyether esters obtained in the melt according to the state of the art as described in U.S. Pat. No. 4,205,158-A.
The process according to the invention for the preparation of a copolyether ester with hard polyester segments of repeating units derived from at least one alkylene glycol and at least one aromatic dicarboxylic acid and soft segments derived from at least one polyalkylene oxide glycol, which comprises polymerization by polycondensation in the melt of at least one aromatic dicarboxylic acid, at least one alkylene glycol and at least one polyalkylene oxide glycol in the presence of a catalyst based on a combination of titanium and a bivalent metal in a single compound or a combination of titanium and bivalent metal containing compounds, is characterized in that the molecular ratio of titanium to bivalent metal is at most approximately 1.6, preferably at most 1.5.
The best results are achieved when the molecular ratio of titanium to bivalent metal is approximately 1.
Within the group of bivalent metals in particular the alkaline earth metals, for instance magnesium, barium and calcium, and zinc are very suitable. Magnesium is preferred. Preferably, the titanium and the bivalent metal are combined in two separate compounds. The compounds already referred to in the introduction are in principle eligible for use in the process according to the invention. However, the invention is not limited to these.
Preferably, the titanium is used in the form of a metal organic compound, for instance in the form of a titanium alkoxide, for instance TBT, or a titanium ester. The bivalent metal is preferably used in the form of a compound that is soluble in the reaction mixture, for instance in the form of an acetate, preferably magnesium acetate. The concentration of the catalyst in the reaction mixture may vary within broad limits; in general the useful activity is within a range of 0.01 wt. %-1 wt. % of TBT, relative to the terephthalic acid or terephthalate used. Preferably, the content lies between 0.03 and 0.3 wt. % TBT. Below a value of 0.01 wt. % TBT no effect is generally noticeable, and at a content of higher than 1 wt. % a polycondensate is obtained that is unsuitable for solid-phase post-condensation. Generally speaking, in the copolymerization of copolyether esters on the basis of polybutylene oxide diol or polyethylene oxide diol a smaller amount of catalyst will suffice than in the copolymerization of copolyether esters on the basis of polypropylene oxide diol. The same holds for the harder copolyether ester types, for which likewise a smaller amount of catalyst needs to be applied than for the softer types.
The titanium containing compound and the bivalent metal containing compound can simultaneously, or optionally separately, be added to the polycondensation. If an ester of the aromatic dicarboxylic acid is used, for instance the dimethyl ester of terephthalic acid, it is sometimes recommendable to add the bivalent metal containing compound only after the transesterification has taken place. The titanium containing compound can then be added in its entirety already at the start of the transesterification reaction in which methanol is released, or in two steps, viz. at the start of the transesterification and at the start of the polycondensation.
The process for the preparation of copolyether esters can otherwise be applied under the customary conditions for melt polycondensation, with the transesterification reaction taking place at elevated temperature, in general first between 150 and 260xc2x0 C., with methanol being distilled off in case the dimethyl ester of terephthalic acid is used, and subsequently the polycondensation being continued at reduced pressure. The pressure is preferably chosen to be between 0.1 and 30 kPa, and the temperature between 230 and 275xc2x0 C.
The polycondensation will be completed in the shortest time at the lowest pressure. It is also possible to use a dry inert gas atmosphere, for instance nitrogen circulation, instead of reduced pressure. Inclusion of oxygen should be avoided.
If desired, the reaction mixture may contain a minor amount of chain branching agent, However, the process according to the invention has the advantage that much lower concentrations than required in U.S. Pat. No. 4,205,258-A suffice. As chain branching agent use can be made of the compounds mentioned in this patent publication, viz. alcohols having a functionality of at least 3, for instance trimethylol propane, pentaerythritol and 1,1,4,4-tetrakis (hydroxymethyl)-cyclohexane, carboxylic acids having a functionality of at least 3, for instance trimellitic acid, trimesinic acid and 1,1,2,2-ethane tetracarboxylic acid and hydrocarboxylic acids having a functionality of at least 3, for instance citric acid, 3-hydroxyglutaric acid and dihydroxyglutaric acid. Preferably, the functionality is 3 or 4. Preferably, use is made of carboxylic acids having a functionality of 3 or 4, for instance trimellitic acid or an ester thereof and trimellitic anhydride. The chain branching agent content is preferably chosen below 0.3 eq/100 moles of dicarboxylic acid, more preferably below 0.2 eq/100 moles.
The process according to the invention is in principle suitable for the preparation of all types of copolyether esters with hard segments of repeating units, derived from at least one alkylene glycol and at least one aromatic dicarboxylic acid or an ester thereof, and soft segments derived from at least one polyalkylene oxide glycol.
The alkylene group generally contains 2-6 carbon atoms, preferably 2-4 C. Preferred alkylene glycols are ethylene glycol, propylene glycol and butylene glycol. As polyalkylene oxide glycol use can be made, for instance, of polybutylene oxide glycol, polypropylene oxide glycol and polyethylene oxide glycol or combinations thereof, for instance ethylene oxide end capped polypropylene oxide glycol. The invention is effective in particular when the polyalkylene oxide glycol is polypropylene oxide glycol or ethylene oxide end capped polypropylene oxide glycol.
Suitable for use as aromatic dicarboxylic acid are terephthalic acid, 1,4-naphthalene dicarboxylic acid, 4,4xe2x80x2-diphenyl dicarboxylic acid. In particular the combinations of butylene glycol or propylene glycol with terephthalic acid or naphthalene dicarboxylic acid and ethylene glycol with naphthalene dicarboxylic acid and diphenyl dicarboxylic acid (molar ratio 6:4-4:6) are very effective as hard segments for copolyether esters with a high melting point. Optionally, other dicarboxylic acids, such as isophthalic acid, may be present. In general, however, the effect of these is to depress the melting point.
The invention will now be elucidated with reference to the following examples and comparative experiments.
Materials used:
aromatic dicarboxylic acid DMT=dimethyl terephthalate
alkylene glycol BDO=butylene glycol
polyalkylene oxide glycol PL6200=Pluronic PE6200(copyright)=polypropylene oxide end capped with ethylene oxide glycol, from BASF Germany. ethylene oxide:propylene oxide=36:64 (weight ratio) THF 2000=tetrahydrofuran of molecular weight=2000
catalyst TBTxe2x80x94titanium tetrabutoxide MgAc=magnesium acetate tetrahydrate
stabilizer Irganox(copyright) 1330 from Ciba-Geigy, Switzerland.
chain branching agent: TMP=trimethylol propane D-TMD=di-trimethylol propane TMA=trimellitic acid TME-TMA=trimethyl ester of trimellitic acid.